1. Field of the Invention
The present invention relates to optical scanners suitable for, for example, electrophotographic image forming apparatuses such as laser beam printers, digital copiers, and multifunction printers, and also relates to image forming apparatuses using the optical scanners. In particular, the present invention relates to an optical scanner in which a light beam emitted from a light source is deflected by a polygon mirror, as a light deflector, to scan a surface to be scanned through a focusing optical system having fθ characteristics so that image information can be recorded, and also relates to an image forming apparatus using the optical scanner.
2. Description of the Related Art
In a conventional optical scanner, a light beam modulated according to image signals is emitted from a light source and is periodically deflected by a light deflector composed of, for example, a rotating polygon mirror. The light beam reflected by the polygon mirror is focused in a spot on the surface of a photosensitive recording medium by a focusing optical system having fθ characteristics to scan the surface for image recording.
In recent years, higher-speed, more compact focusing optical systems have been demanded with speed increases and size reductions being achieved in devices such as laser beam printers, digital multifunction machines, and multifunction printers.
An example of methods for increasing speed is the use of an overfilled optical system (hereinafter also referred to as OFS). In an OFS, each deflecting surface (reflective surface) of the light deflector requires only the same width as the substantial part of the incident light beam required for deflection scanning. The light deflector can therefore have a smaller diameter and more surfaces. Accordingly, the OFS is suitable for increasing speed.
The OFS, however, has the problems described below.
In the OFS, a light beam wider than the deflecting surface of the light deflector in a main scanning direction is made incident on the deflecting surface. In deflection, the portion of the light beam incident on the deflecting surface is separated and guided to a surface to be scanned. Thus, different portions of the light beam incident on the deflecting surface are used at different image heights on the surface to be scanned. For example, a light beam guided to the center of the surface to be scanned is the central portion of the light beam incident on the light deflector, and a light beam guided to a marginal area (off-axis image height) of the surface to be scanned is a marginal portion of the light beam incident on the light deflector.
If, therefore, a difference in wavefront shape, such as spherical aberration, occurs between the central portion and marginal portions of the light beam incident on the light deflector, the wavefronts of light beams guided to the marginal areas (off-axis image height) of the surface to be scanned are asymmetrical in the main scanning direction. FIG. 11 shows an example of a wavefront aberration in the light beam incident on the light deflector in the OFS.
FIG. 12A shows a wavefront aberration in a light beam separated and deflected by the light deflector at an off-axis image height in a known OFS (a wavefront aberration occurring in an incident optical system). FIG. 12B shows a wavefront aberration occurring in a focusing optical system in the OFS. In the OFS, the wavefront aberration occurring in the focusing optical system is corrected so that no wavefront aberration remains in the focusing optical system, as shown in FIG. 12B. Consequently, a wavefront aberration that is asymmetrical in the main scanning direction occurs in the overall system at the off-axis image height, as shown in FIG. 12C. That is, the known OFS disadvantageously causes coma aberration in the overall system at an off-axis image height due to, for example, spherical aberration in the incident optical system.
In addition, a focusing optical system composed of a single lens or having at least one surface having an arc shape in the main scanning direction is advantageous in terms of ease of manufacture, though a known OFS including such a focusing optical system has difficulty in completely inhibiting coma aberration occurring in the focusing optical system at all image heights. In this case, unfortunately, the known OFS causes coma aberration in the overall system because the direction of the aberration occurring in the focusing optical system is the same as that of the aberration occurring in the incident optical system.
FIGS. 13A, 13B, and 13C show wavefront aberrations caused in the main scanning direction at an off-axis image height by the incident optical system, the focusing optical system, and the overall system, respectively, in a known OFS including a focusing optical system composed of a single lens.
FIGS. 13A and 13B show that the direction of coma aberration due to, for example, spherical aberration in the incident optical system at an off-axis image height is the same as that of coma aberration that cannot be inhibited in the focusing optical system at the off-axis image height. The wavefront aberrations occurring in the incident optical system and the focusing optical system at the off-axis image height combine with each other to cause coma aberration in the overall system at the off-axis image height, as shown in FIG. 13C.
Coma aberrations as shown in FIGS. 12C and 13C tend to cause a spot with a side lobe on a surface to be scanned. FIG. 14 is a diagram illustrating a spot profile at an off-axis image height in a known OFS. FIG. 14 shows that a side lobe occurs in the main scanning direction.
In FIGS. 12C and 13C, additionally, the wavefront aberrations at the off-axis image height in the known OFS are curved. The focal position for the curved wavefront aberrations deviates from that for a reference sphere depending on the amount of curvature of the wavefront aberrations. Because the amount of curvature varies with the image height in the known OFS, a difference in focal position between image heights, namely field curvature, occurs on the surface to be scanned, thus disadvantageously expanding the diameter of beam spots.
Side lobes and expanded beam spots may have adverse effects on images written on the surface to be scanned, such as decreased resolution and expanded fine lines.
Various optical scanners have been proposed to solve the above problems.
According to Japanese Patent Laid-Open No. 2001-59946 (no corresponding foreign publication), a collimating lens part for collimating a light beam emitted from a light source is composed of a plurality of lenses or an aspherical lens to inhibit spherical aberration due to the collimating lens itself and thus excellently correct field curvature.
According to the above publication, however, the number of lenses used for the collimating lens part must be increased to inhibit the spherical aberration at the collimating lens part. Alternatively, the accuracy of the surface shape and attachment of the aspherical lens used must be improved. The use of the aspherical lens therefore tends to result in a complicated (costly) incident optical system. In particular, a larger spherical aberration occurs as the F-number (Fno) on the incident side of a focusing optical system in the main scanning direction is reduced to increase coupling efficiency and thus achieve a higher scanning speed. The number of lenses used must therefore be increased to inhibit the spherical aberration. This tends to result in a complicated (costly) incident optical system.
A double-pass structure is employed to provide a compact focusing optical system. In this structure, both a light beam incident on a deflecting surface of a light deflector and a light beam deflected by the deflecting surface pass through at least one of the lenses constituting a focusing optical system. The double-pass structure also causes aberration when a light beam traveling toward the light deflector passes through the lens. In particular, a larger wavefront aberration tends to occur if the focusing optical system (fθ lens) has a non-arc generating line in the main scanning direction to reduce the optical pass length of the focusing optical system. The double-pass structure therefore has difficulty in completely correcting the wavefront aberration occurring in the incident optical system in the main scanning direction. This tends to cause difficulty in providing excellent spots.
According to the above publication, additionally, the focusing optical system is composed of a single fθ lens because such a system is advantageous in terms of ease of manufacture. In this case, however, the focusing optical system has difficulty in inhibiting the aberration due to the system itself and, for example, no consideration is given to the occurrence of coma aberration. Consequently, even if the spherical aberration in the incident optical system is completely inhibited, coma aberration that cannot be inhibited in the focusing optical system leads to coma aberration in the overall system. This optical scanner therefore has a tendency to fail to provide excellent spots.
U.S. Pat. No. 5,757,535 (Japanese Patent Laid-Open No. 9-304720) is aimed at reducing a side lobe, which is caused by the asymmetrical light intensity distribution in the main scanning direction of a light beam deflected and separated by a deflecting surface of a light deflector at an off-axis image height. According to this publication, a side lobe is reduced by replacing a collimating lens with an aspherical optical component that allows a light beam to exit with the wavefront thereof deviating from a reference sphere with increasing height from the optical axis.
According to the above publication, however, no consideration is given to the asymmetry of the wavefront aberration of a light beam guided to an off-axis image height (marginal areas) on a surface to be scanned; the wavefront aberration is due to, for example, spherical aberration in the incident optical system. If, therefore, an asymmetrical wavefront aberration as shown in FIG. 12A occurs in the incident optical system, the asymmetrical optical component has difficulty in reducing a side lobe. FIG. 14 shows a spot profile in this case. In FIG. 14, the side lobe is not reduced. Unfortunately, therefore, this optical scanner cannot provide excellent spots.
According to the above publication, additionally, coma aberration occurs in the overall system because no consideration is given to coma aberration occurring in the focusing optical system which cannot be completely inhibited by the focusing optical system itself. Unfortunately, therefore, this optical scanner cannot reduce a side lobe and provide excellent spots.
Furthermore, field curvature cannot be completely inhibited because no consideration is given to field curvature due to differences in the amount of curvature of wavefront aberration at off-axis image heights. The diameter of spots therefore undesirably varies at off-axis image heights.